1. Field of the Invention
The present invention relates to a trans-impedance amplifier (hereafter denoted as TIA), in particular, the invention relates to the TIA with the function of the variable bandwidth and a digital coherent receiver installing the TIA.
2. Related Prior Art
A TIA used in an optical receiver may convert a photocurrent generated by a light-receiving device, for instance, a photodiode (hereafter denoted as PD) into a voltage signal, and sometimes amplifies thus converted voltage signal. A future system featured by the digital coherent technology may install a plurality of TIAs. U.S. Pat. No. 7,358,818, has disclosed a pre-amplifier of the type of the TIA. The resistance/impedance of the trans-impedance, or the open loop gain of the amplifier of the TIA disclosed therein may adjust the bandwidth of the TIA.
In an arrangement where the trans-impedance is so adjusted to equalize the bandwidth of the TIAs in the coherent receiver, the trans-impedance may also vary other factors of the TIA such as the phase-gain tolerance thereof, which may degrade the signal quality output from the TIA. The digital coherent receiver applied to the system using the phase shift keying (PSK) modulates the phase of the input signal; accordingly, the phase characteristic of the TIA is preferable to be independent of the bandwidth thereof.